Liquid-crystalline medium

ABSTRACT

The invention relates to a liquid-crystalline medium based on a mixture of polar compounds of positive dielectric anisotropy, characterized in that it comprises one or more compounds of the formula I                    
     in which R 1 , L 1  and L 2  are as defined in claim  1.

The present invention relates to a liquid-crystalline medium, to the usethereof for electro-optical purposes, and to displays containing thismedium.

Liquid-crystals are used principally as dielectrics in display devices,since the optical properties of such substances can be modified by anapplied voltage. Electro-optical devices based on liquid crystals areextremely well known to the person skilled in the art and can be basedon various effects. Examples of such devices are cells having dynamicscattering, DAP (deformation of aligned phases) cells, guest/host cells,TN cells having a twisted nematic structure, STN (supertwisted nematic)cells, SBE (superbirefringence effect) cells and OMI (optical modeinterference) cells. The commonest display devices are based on theSchadt-Helfrich effect and have a twisted nematic structure.

The liquid-crystal materials must have good chemical and thermalstability and good stability to electric fields and electromagneticradiation. Furthermore, the liquid-crystal materials should have lowviscosity and produce short addressing times, low threshold voltages andhigh contrast in the cells.

They should furthermore have a suitable mesophase, for example a nematicor cholesteric mesophase for the above-mentioned cells, at the usualoperating temperatures, i.e. in the broadest possible range above andbelow room temperature. Since liquid crystals are generally used asmixtures of a plurality of components, it is important that thecomponents are readily miscible with one another. Further properties,such as the electrical conductivity, the dielectric anisotropy and theoptical anisotropy, have to satisfy various requirements depending onthe cell type and area of application. For example, materials for cellshaving a twisted nematic structure should have positive dielectricanisotropy and low electrical conductivity.

For example, for matrix liquid-crystal displays with integratednon-linear elements for switching individual pixels (MLC displays),media having large positive dielectric anisotropy, broad nematic phases,relatively low birefringence, very high specific resistance, good UV andtemperature stability and lower vapor pressure are desired.

Matrix liquid-crystal displays of this type are known. Non-linearelements which can be used for individual switching of the individualpixels are, for example, active elements (i.e. transistors). The term“active matrix” is then used, where a distinction can be made betweentwo types:

1. MOS (metal oxide semiconductor) or other diodes on a silicon wafer assubstrate.

2. Thin-film transistors (TFTs) on a glass plate as substrate.

The use of single-crystal silicon as substrate material restricts thedisplay size, since even modular assembly of various part-displaysresults in problems at the joins.

In the case of the more promising type 2, which is preferred, theelectro-optical effect used is usually the TN effect. A distinction ismade between two technologies: TFTs comprising compound semiconductors,such as, for example, CdSe, or TFTs based on polycrystalline oramorphous silicon. Intensive work is being carried out world-wide on thelatter technology.

The TFT matrix is applied to the inside of one glass plate of thedisplay, while the other glass plate carries the transparentcounterelectrode on its inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fully color-capabledisplays, in which a mosaic of red, green and blue filters is arrangedin such a way that a filter element is opposite each switchable pixel.

The TFT displays usually operate as TN cells with crossed polarizers intransmission and are illuminated from the back.

The term MLC displays here covers any matrix display with integratednon-linear elements, i.e., besides the active matrix, also displays withpassive elements, such as varistors or diodes(MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications(for example pocket TVs) or for high-information displays for computerapplications (laptops) and in automobile or aircraft construction.Besides problems regarding the angle dependence of the contrast and theresponse times, difficulties also arise in MLC displays due toinsufficiently high specific resistance of the liquid-crystal mixtures[TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIUMACHI, K.,TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p.141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Designof Thin Film Transistors for Matrix Addressing of Television LiquidCrystal Displays, p. 145 ff, Paris]. With decreasing resistance, thecontrast of an MLC display deteriorates, and the problem of after-imageelimination may occur. Since the specific resistance of theliquid-crystal mixture generally drops over the life of an MLC displayowing to interaction with the interior surfaces of the display, a high(initial) resistance is very important in order to obtain acceptableservice lives. In particular in the case of low-volt mixtures, it washitherto impossible to achieve very high specific resistance values. Itis furthermore important that the specific resistance exhibits thesmallest possible increase with increasing temperature and after heatingand/or UV exposure. The low-temperature properties of the mixtures fromthe prior art are also particularly disadvantageous. It is demanded thatno crystallization and/or smectic phases occur, even at lowtemperatures, and the temperature dependence of the viscosity is as lowas possible. The MLC displays from the prior art thus do not meettoday's requirements.

There thus continues to be a great demand for MLC displays having veryhigh specific resistance at the same time as a large working-temperaturerange, short response times even at low temperatures and low thresholdvoltage which do not have these disadvantages, or only do so to areduced extent.

In TN (Schadt-Helfrich) cells, media are desired which facilitate thefollowing advantages in the cells:

extended nematic phase range (in particular down to low temperatures)

stable on storage, even at low temperatures

the ability to switch at extremely low temperatures (outdoor use,automobile, avionics)

increased resistance to UV radiation (longer service life).

The media available from the prior art do not allow these advantages tobe achieved while simultaneously retaining the other parameters.

In the case of supertwisted (STN) cells, media are desired which enablegreater multiplexability and/or lower threshold voltages and/or broadernematic phase ranges (in particular at low temperatures). To this end, afurther widening of the available parameter latitude (clearing point,smectic-nematic transition or melting point, viscosity, dielectricparameters, elastic parameters) is urgently desired.

The invention has an object of providing media, in particular for MLC,TN or STN displays of this type, which do not have the above-mentioneddisadvantages or only do so to a reduced extent, and preferablysimultaneously have very high specific resistance values and lowthreshold voltages.

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

It has now been found that these and other objects can be achieved ifmedia according to the invention are used in displays.

The invention thus relates to a liquid-crystalline medium based on amixture of polar compounds of positive dielectric anisotropy,characterized in that it comprises one or more compounds of the formulaI

in which

R¹ is a halogenated or unsubstituted alkyl or alkoxy radical having from1 to 15 carbon atoms, where, in addition, one or more CH₂ groups inthese radicals may each, independently of one another, be replaced by—C≡C—, —CH═CH—, —O—, —CO—O— or —O—CO— in such a way that O atoms are notlinked directly to one another,

X is F, Cl, CN, SF₅, a halogenated alkyl radical, a halogenated alkenylradical, a halogenated alkoxy radical or a halogenated alkenyloxyradical having up to 6 carbon atoms,

L¹ and L² are each, independently of one another, H or F.

The compounds of the formula I have a broad range of applications.Depending on the choice of substituents, these compounds can serve asbase materials of which liquid-crystalline media are predominantlycomposed; however, it is also possible to add compounds of the formula Iliquid-crystalline base materials from other classes of compound inorder, for example, to modify the dielectric and/or optical anisotropyof a dielectric of this type and/or in order to optimize its thresholdvoltage and/or its viscosity.

In the pure state, the compounds of the formula I are colorless and formliquid-crystalline mesophases in a temperature range which is favorablylocated for electro-optical use. They are stable chemically, thermallyand to light.

If R¹ in the formula I is an alkyl radical and/or an alkoxy radical,this may be straight-chain or branched. It is preferably straight-chain,has 2, 3, 4, 5, 6 or 7 carbon atoms and accordingly is preferably ethyl,propyl, butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy,hexyloxy or heptyloxy, furthermore methyl, octyl, nonyl, decyl, undecyl,dodecyl, tridecyl, tetradecyl, pentadecyl, methoxy, octyloxy, nonyloxy,decyloxy, undecyloxy, dodecyloxy, tridecyloxy or tetradecyloxy.

Oxaalkyl is preferably straight-chain 2-oxapropyl (=methoxymethyl),2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl,2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5- , 6-, 7- or 8-oxanonyl,or 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.

If R¹ is an alkyl radical in which one CH₂ group has been replaced by—CH═CH—, this may be straight-chain or branched. It is preferablystraight-chain and has 2 to 10 carbon atoms. Accordingly, it is inparticular vinyl, prop-1- or prop-2-enyl, but-1-, -2- or -3-enyl,pent-1-, -2-, -3- or -4-enyl, hex-1-, -2-, -3-, -4- or -5-enyl, hept-1-,-2-, -3-, -4-, -5- or -6-enyl, oct-1-, -2-, -3-, -4-, -5-, -6- or-7-enyl, non-1-, -2-, -3-, -4-, -5-, -6-, -7- or -8-enyl, or dec-1-,-2-, -3-, -4-, -5-, -6-, -7-, -8- or -9-enyl.

If R¹ is an alkyl radical in which one CH₂ group has been replaced by—O— and one has been replaced by —CO—, these are preferably adjacent.These thus contain an acyloxy group —CO—O— or an oxycarbonyl group—O—CO. These are preferably straight-chain and have 2 to 6 carbon atoms.Accordingly, they are in particular acetoxy, propionyloxy, butyryloxy,pentanoyloxy, hexanoyloxy, acetoxymethyl, propionyloxymethyl,butyryloxymethyl, pentanoyloxymethyl, 2-acetoxyethyl,2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetoxypropyl,3-propionyloxypropyl, 4-acetoxybutyl, methoxycarbonyl, ethoxycarbonyl,propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl,ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl,2-(methoxycarbonyl)ethyl, 2-(ethoxycarbonyl)ethyl,2-(propoxycarbonyl)ethyl, 3-(methoxycarbonyl)propyl,3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.

If R¹ is an alkyl radical in which one CH₂ group has been replaced byunsubstituted or substituted —CH═CH— and an adjacent CH₂ group has beenreplaced by CO or CO—O or O—CO, this may be straight-chain or branched.It is preferably straight-chain and has 4 to 12 carbon atoms.Accordingly, it is in particular acryloyloxymethyl, 2-acryloyloxyethyl,3-acryloyloxypropyl, 4-acryloyloxybutyl, 5-acryloyloxypentyl,6-acryloyloxyhexyl, 7-acryl-oyloxyheptyl, 8-acryloyloxyoctyl,9-acryloyloxynonyl, 10-acryloyloxydecyl, methacryloyloxymethyl,2-methacryloyloxyethyl, 3-methacryloyloxypropyl,4-meth-acryloyloxybutyl, 5-methacryloyloxypentyl,6-methacryloyloxyhexyl, 7-methacryloyloxyheptyl, 8-methacryloyloxyoctylor 9-methacryloyloxynonyl.

If R¹ is an alkyl or alkenyl radical which is monosubstituted by CN orCF₃, this radical is preferably straight-chain. The substitution by CNor CF₃ is in any desired position.

If R¹ is an alkyl or alkenyl radical which is at least monosubstitutedby halogen, this radical is preferably straight-chain, and halogen ispreferably F or Cl. In the case of polysubstitution, halogen ispreferably F. The resultant radicals also include perfluorinatedradicals. In the case of monosubstitution, the fluorine or chlorinesubstituent may be in any desired position, but is preferably in theco-position.

Compounds containing branched wing groups R¹ may occasionally be ofimportance owing to better solubility in the conventionalliquid-crystalline base materials, but in particular as chiral dopantsif they are optically active. Smectic compounds of this type aresuitable as components of ferroelectric materials.

Branched groups of this type generally contain not more than one chainbranch. Preferred branched radicals R¹ are isopropyl, 2-butyl(=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl(=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl,2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy,3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexyloxy,1-methylhexyloxy and 1-methylheptyloxy.

If R¹ is an alkyl radical in which two or more CH₂ groups have beenreplaced by —O— and/or —CO—O—, this may be straight-chain or branched.It is preferably branched and has from 3 to 12 carbon atoms.Accordingly, it is in particular biscarboxymethyl, 2,2-biscarboxyethyl,3,3-biscarboxypropyl, 4,4-biscarboxybutyl, 5,5-biscarboxypentyl,6,6-biscarboxyhexyl, 7,7-biscarboxyheptyl, 8,8-biscarboxyoctyl,9,9-biscarboxynonyl, 10,10-biscarboxydecyl, bis(methoxycarbonyl)methyl,2,2-bis(methoxycarbonyl)ethyl, 3,3-bis(methoxycarbonyl)propyl,4,4-bis(methoxycarbonyl)butyl, 5,5-bis(methoxycarbonyl)pentyl,6,6-bis(methoxycarbonyl)hexyl, 7,7-bis(methoxycarbonyl)heptyl,8,8-bis(methoxycarbonyl)octyl, bis(ethoxycarbonyl)methyl,2,2-bis(ethoxycarbonyl)ethyl, 3,3-bis(ethoxycarbonyl)propyl,4,4-bis(ethoxycarbonyl)butyl or 5,5-bis(ethoxycarbonyl)hexyl.

The compounds of the formula I are prepared by methods known per se, asdescribed in the literature (for example in the standard works, such asHouben-Weyl, Methoden der organischen Chemie [Methods of OrganicChemistry], Georg-Thieme-Verlag, Stuttgart), to be precise underreaction conditions which are known and suitable for the said reactions.Use can also be made here of variants which are known per se, but arenot mentioned here in greater detail. The compounds of the formula I canbe prepared, for example, as described in DE 4006921, WO 01/64667 and DE10105314.

The invention also relates to electro-optical displays (in particularSTN or MLC displays having two plane-parallel outer plates, which,together with a frame, form a cell, integrated non-linear elements forswitching individual pixels on the outer plates, and a nematicliquid-crystal mixture of positive dielectric anisotropy and highspecific resistance which is located in the cell) which contain media ofthis type, and to the use of these media for electro-optical purposes.

The liquid-crystal mixtures according to the invention enable asignificant widening of the available parameter latitude.

The achievable combinations of clearing point, viscosity at lowtemperature, thermal and UV stability and dielectric anisotropy are farsuperior to previous materials from the prior art.

The requirement for a high clearing point, a nematic phase at lowtemperature and a high Δε has hitherto only been achieved to aninadequate extent. Although mixtures such as, for example, ZLI-3119 havea comparable clearing point and comparably favorable viscosities, theyhave, however, a Δε of only +3. Other mixture systems have comparableviscosities and Δε values, but only have clearing points in the regionof 60° C.

The liquid-crystal mixtures according to the invention, for examplewhile retaining the nematic phase down to −20° C. and preferably down to−30° C., particularly preferably down to −40° C., preferably enable aclearing point above 60° C., more preferably above 65° C., particularlypreferably above 70° C., simultaneously dielectric anisotropy values Δεpreferably of ≧6, more preferably ≧8, and a high value for the specificresistance to be achieved, enabling excellent STN and MLC displays to beobtained. In particular, the mixtures are characterized by low operatingvoltages. The TN thresholds are preferably below 2.0 V, more preferablybelow 1.7 V, particularly preferably <1.3 V.

It goes without saying that, through a suitable choice of the componentsof the mixtures according to the invention, it is also possible forhigher clearing points (for example above 110° C.) to be achieved at ahigher threshold voltage or lower clearing points to be achieved atlower threshold voltages with retention of the other advantageousproperties. At viscosities correspondingly increased only slightly, itis likewise possible to obtain mixtures having greater Δε and thus lowerthresholds. The MLC displays according to the invention preferablyoperate at the first Gooch and Tarry transmission minimum [C. H. Goochand H. A. Tarry, Electron. Lett. 10, 2-4, 1974; C. H. Gooch and H. A.Tarry, Appl. Phys., Vol. 8, 1575-1584, 1975] are used, where, besidesparticularly favourable electro-optical properties, such as, forexample, high steepness of the characteristic line and low angledependence of the contrast (German Patent 30 22 818), a lower dielectricanisotropy is sufficient at the same threshold voltage as in ananalogous display at the second minimum. This enables significantlyhigher specific resistances to be achieved using the mixtures accordingto the invention at the first minimum than in the case of mixturescomprising cyano compounds. Through a suitable choice of the individualcomponents and their proportions by weight, the person skilled in theart is able to set the birefringence necessary for a pre-specified layerthickness of the MLC display using simple routine methods.

The flow viscosity ν₂₀ at 20° C. is preferably <60 mm²·s⁻¹, particularlypreferably <50 mm²·s⁻¹. The rotational viscosity γ₁ at 20° C. of themixtures according to the invention is preferably <160 mPa·s,particularly preferably <150 mPa·s. The nematic phase range ispreferably at least 90°, in particular at least 100°. This rangepreferably extends at least from −20° to +80° C.

A short response time is desired in liquid-crystal displays. Thisapplies in particular to displays which are capable of videoreproduction. For displays of this type, response times (total:t_(on)+t_(off)) of at most 25 ms are required. The upper limit of theresponse time is determined by the image refresh frequency. Besides therotational viscosity γ₁, the tilt angle likewise affects the responsetime. In particular, mixtures comprising ≧20% of the compounds of theformula I exhibit a tilt angle of >2.5, preferably >3.0, compared withthe commercial product ZLI-4792 from Merck KGaA.

Measurements of the voltage holding ratio (HR) [S. Matsumoto et al.,Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SID Conference,San Francisco, June 1984, p. 304 (1984); G. Weber et al., LiquidCrystals 5, 1381 (1989)] have shown that mixtures according to theinvention comprising compounds of the formula I exhibit a significantlysmaller decrease in the HR with increasing temperature than, forexample, analogous mixtures comprising cyanophenylcyclohexanes of theformula

or esters of the formula

instead of the compounds of the formula I.

The UV stability of the mixtures according to the invention is alsoconsiderably better, i.e. they exhibit a significantly smaller decreasein the HR on exposure to UV.

Particularly preferred compounds of the formula I are compounds of theformulae I-1 to I-9:

in which R¹ is as defined in the formula I.

Of these preferred compounds, particular preference is given to those ofthe formulae I-1, I-2, I-3 and I-4, in particular those of the formulaeI-1 and 1-2.

Preferred embodiments are indicated below:

the medium comprises one, two or more compounds of the formulae I-1 to1-9;

the medium additionally comprises one or more compounds selected fromthe group consisting of the general formulae IL to VI:

in which the individual radicals have the following meanings:

R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having up to 9carbon atoms,

X⁰ is F, Cl, halogenated alkyl, halogenated alkenyl, halogenatedalkenyloxy or halogenated alkoxy having up to 6 carbon atoms,

Z⁰ is —C₂F₄—, —CF═CF—, —C₂H₄—, —(CH₂)₄—, —OCH₂—, —CH₂O—, —CF₂O— or—OCF₂—,

Y¹ to Y⁴ are each, independently of one another, H or F,

r is 0 or 1.

The compound of the formula IV is preferably

the medium additionally comprises one or more compounds selected fromthe group consisting of the general formulae VII to XIII:

in which R⁰, X⁰ and Y¹⁻⁴ are each, independently of one another, asdefined herein. X⁰ is preferably F, Cl, CF₃, OCF₃ or OCHF₂. R⁰ ispreferably alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having up to 6carbon atoms.

The medium additionally comprises one or more compounds of the formulaeE-a to E-d

in which R⁰ is as defined herein;

the proportion of the compounds of the formulae E-a to E-d is preferably10-30% by weight, in particular 15-25% by weight;

the proportion of compounds of the formulae I to VI together in themixture as a whole is at least 50% by weight;

the proportion of compounds of the formula I in the mixture as a wholeis from 0.5 to 40% by weight, particularly preferably from 1 to 30% byweight;

the proportion of compounds of the formulae II to VI in the mixture as awhole is from 30 to 80% by weight;

the medium comprises compounds of the formulae II, III, IV, V and/or VI;

R⁰ is straight-chain alkyl or alkenyl having from 2 to 7 carbon atoms;

the medium essentially consists of compounds of the formulae I to VI andXIII;

the medium comprises further compounds, preferably selected from thefollowing group consisting of the general formulae XIV to XVIII:

in which R⁰ and X⁰ are as defined above. The 1,4-phenylene rings mayadditionally be substituted by CN, chlorine or fluorine. The1,4-phenylene rings are preferably monosubstituted or polysubstituted byfluorine atoms.

The medium additionally comprises one, two, three or more, preferablytwo or three, compounds of the formulae

in which “alkyl” and “alkyl*” are as defined below. The proportion ofthe compounds of the formulae O1 and/or O2 in the mixtures according tothe invention is preferably 5-10% by weight.

The medium preferably comprises 5-35% by weight of compound IVa.

The medium preferably comprises one, two or three compounds of theformula IVa in which X⁰ is F or OCF₃.

The medium preferably comprises one or more compounds of the formulaeIIa to IIg

in which R⁰ is as defined above. In the compounds of the formulaeIIa-IIg, R⁰ is preferably methyl, ethyl, n-propyl, n-butyl or n-pentyl.

The medium preferably comprises one or more compounds of the formulae

in which R⁰ is as defined above.

The (I):II+III+IV+V+VI) ratio by weight is preferably from 1:10 to 10:1.

The medium essentially consists of compounds selected from the groupconsisting of the general formulae I to XIII.

The proportion of the compounds of the formula IVb and/or IVc in whichX⁰ is fluorine and R⁰ is CH₃, C₂H₅, n-C₃H₇, n-C₄H₉ or n-C₅H₁₁ in themixture as a whole is from 2 to 20% by weight, in particular from 2 to15% by weight.

The medium preferably comprises compounds of the formulae II to VI inwhich R⁰ is methyl.

The medium particularly preferably comprises compounds of the formulae

The medium preferably comprises one, two or more, preferably one or two,dioxane compounds of the formulae

The medium additionally comprises one, two or more bicyclic compounds ofthe formulae Z1 to Z6

in which R^(1a) and R^(2a) are each, independently of one another, H,CH₃, C₂H₅ or n-C₃H₇. R⁰, alkyl and alkyl* are as defined in claim 3 oras defined below.

Of the said bicyclic compounds, particular preference is given to thecompounds Z-1, Z-2, Z-5 and Z-6.

The medium additionally comprises one, two or more compounds havingfused rings, of the formulae AN1to AN11:

in which R⁰ is as defined above.

It has been found that even a relatively small proportion of compoundsof the formulae I mixed with conventional liquid-crystal materials, butin particular with one or more compounds of the formulae II, III, IV, Vand/or VI, results in a lowering of the threshold voltage and in lowbirefringence values, with broad nematic phases with low smectic-nematictransition temperatures being observed at the same time, improving theshelf life. Preference is given, in particular, to mixtures which,besides one or more compounds of the formulae I, comprise one or morecompounds of the formula IV, in particular compounds of the formula IVain which X⁰ is F or OCF₃. The compounds of the formulae I to VI arecolorless, stable and readily miscible with one another and with otherliquid-crystalline materials.

The term “alkyl” or “alkyl*” covers straight-chain and branched alkylgroups having 1-7 carbon atoms, in particular the straight-chain groupsmethyl, ethyl, propyl, butyl, pentyl, hexyl and heptyl. Groups having1-5 carbon atoms are generally preferred.

The term “alkenyl” covers straight-chain and branched alkenyl groupshaving 2-7 carbon atoms, in particular the straight-chain groups.Preferred alkenyl groups are C₂-C₇1E-alkenyl, C₄-C₇3E-alkenyl,C₅-C₇4-alkenyl, C₆-C₇5-alkenyl and C₇-6-alkenyl, in particularC₂-C₇1E-alkenyl, C₄-C₇3E-alkenyl and C₅-C₇4-alkenyl. Examples of partpreferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl,1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl,3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl,4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5carbon atoms are generally preferred.

The term “fluoroalkyl” preferably covers straight-chain groups having aterminal fluorine, i.e. fluoromethyl, 2-fluoroethyl, 3-fluoropropyl,4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and 7-fluoroheptyl.However, other positions of the fluorine are not excluded.

The term “oxaalkyl” preferably covers straight-chain radicals of theformula C_(n)H_(2n+1)—O—(CH₂)_(m), in which n and m are each,independently of one another, from 1 to 6. Preferably, n=1 and m is from1 to 6.

Through a suitable choice of the meanings of R⁰ and X⁰, the addressingtimes, the threshold voltage, the steepness of the transmissioncharacteristic lines, etc., can be modified in the desired manner. Forexample, 1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxyradicals and the like generally result in shorter addressing times,improved nematic tendencies and a higher ratio of the elastic constantsk₃₃ (bend) and k₁₁ (splay) compared with alkyl or alkoxy radicals.4-alkenyl radicals, 3-alkenyl radicals and the like generally give lowerthreshold voltages and smaller values of k₃₃/k₁₁ compared with alkyl andalkoxy radicals.

A —CH₂CH₂— group generally results in higher values of k₃₃/k₁₁ comparedwith a single covalent bond. Higher values of k₃₃/k₁₁ facilitate, forexample, flatter transmission characteristic lines in TN cells with a90° twist (in order to achieve grey shades) and steeper transmissioncharacteristic lines in STN, SBE and OMI cells (greatermultiplexability), and vice versa.

The optimum mixing ratio of the compounds of the formulae I andII+m+IV+V+VI depends substantially on the desired properties, on thechoice of the components of the formulae I, II, III, IV, V and/or VI,and on the choice of any other components that may be present. Suitablemixing ratios within the range given above can easily be determined fromcase to case.

The total amount of compounds of the formulae I to XIII in the mixturesaccording to the invention is not crucial. The mixtures can thereforecomprise one or more further components for the purposes of optimizationof various properties. However, the observed effect on the addressingtimes and the threshold voltage is generally greater, the higher thetotal concentration of compounds of the formulae I to XIII.

In a particularly preferred embodiment, the media according to theinvention comprise compounds of the formulae II to VI (preferably II,III and/or IV, in particular IVa) in which X⁰ is F, OCF₃, OCHF₂,OCH═CF₂, OCF═CF₂ or OCF₂—CF₂H. A favorable synergistic effect with thecompounds of the formulae I results in particularly advantageousproperties. In particular, mixtures comprising compounds of the formulaI and of the formula IVa are distinguished by their low thresholdvoltages.

The individual compounds of the formulae I to XVIII and theirsub-formulae which can be used in the media according to the inventionare either known or can be prepared analogously to the known compounds.

The construction of the MLC display according to the invention frompolarizers, electrode base plates and surface-treated electrodescorresponds to the conventional construction for displays of this type.The term conventional construction is broadly drawn here and also coversall derivatives and modifications of the MLC display, in particularincluding matrix display elements based on poly-Si TFT or MIM.

A significant difference between the displays according to the inventionand the conventional displays based on the twisted nematic cellconsists, however, in the choice of the liquid-crystal parameters of theliquid-crystal layer.

The liquid-crystal mixtures which can be used in accordance with theinvention are prepared in a manner conventional per se. In general, thedesired amount of the components used in the lesser amount is dissolvedin the components making up the principal constituent, advantageously atelevated temperature. It is also possible to mix solutions of thecomponents in an organic solvent, for example in acetone, chloroform ormethanol, and to remove the solvent again, for example by distillation,after thorough mixing.

The dielectrics may also comprise further additives known to the personskilled in the art and described in the literature. For example, 0-15%of pleochroic dyes or chiral dopants can be added.

C denotes a crystalline phase, S a smectic phase, S_(C) a smectic Cphase, N a nematic phase and I the isotropic phase.

V₁₀ denotes the voltage for 10% transmission (viewing angleperpendicular to the plate surface). t_(on) denotes the switch-on timeand t_(off) the switch-off time at an operating voltage corresponding to2.0 times the value of V₁₀. Δn denotes the optical anisotropy. Δεdenotes the dielectric anisotropy (Δε=ε∥−ε⊥, where ε∥ denotes thedielectric constant parallel to the longitudinal molecular axes and ε⊥denotes the dielectric constant perpendicular thereto). Theelectro-optical data are measured in a TN cell at the 1st minimum (i.e.at a d·Δn value of 0.5 μm) at 20° C., unless expressly stated otherwise.The optical data are measured at 20° C., unless expressly statedotherwise.

In the present application and in the examples below, the structures ofthe liquid-crystal compounds are indicated by means of acronyms, thetransformation into chemical formulae taking place in accordance withTables A and B below. All radicals C_(n)H_(2n+1) and C_(m)H_(2m+1) arestraight-chain alkyl radicals having n and m carbon atoms respectively;n and m are integers and are preferably 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,10, 11 or 12. The coding in Table B is self-evident. In Table A, onlythe acronym for the parent structure is indicated. In individual cases,the acronym for the parent structure is followed, separated by a dash,by a code for the substituents R^(1*), R^(2*), L^(1*) and L^(2*):

Code for R¹*, R²*, L¹*, L²* R¹* R²* L¹* L²* nm C_(n)H_(2n+1)C_(m)H_(2m+1) H H nOm OC_(n)H_(2n+1) C_(m)H_(2m+1) H H nO.mC_(n)H_(2n+1) OC_(m)H_(2m+1) H H n C_(n)H_(2n+1) CN H H nN.FC_(n)H_(2n+1) CN F H nN.F.F C_(n)H_(2n+1) CN F F nF C_(n)H_(2n+1) F H HnCl C_(n)H_(2n+1) Cl H H nOF OC_(n)H_(2n+1) F H H nF.F C_(n)H_(2n+1) F FH nF.F.F C_(n)H_(2n+1) F F F nmF C_(n)H_(2n+1) C_(m)H_(2m+1) F H nOCF₃C_(n)H_(2n+1) OCF₃ H H nOCF₃.F C_(n)H_(2n+1) OCF₃ F H n-Vm C_(n)H_(2n+1)—CH═CH—C_(m)H_(2m+1) H H nV-Vm C_(n)H_(2n+1)— —CH═CH—C_(m)H_(2m+1) H HCH═CH—

Preferred mixture components are given in Tables A and B.

TABLE A

PYP

PYRP

BCH

CBC

CCH

CCP

CPTP

CEPTP

ECCP

CECP

EPCH

PCH

PTP

BECH

EBCH

CPC

B

FET-nF

CGG

CGU

CFU

TABLE B

BCH-n.Fm

CFU-n-F

CBC-nmF

ECCP-nm

CCZU-n-F

T-nFm

CGU-n-F

CDU-n-F

DCU-n-F

CGG-n-F

CPZG-n-OT

CC-nV-Vm

CCP-Vn-m

CCG-V-F

CCP-nV-m

CC-n-V

CCQU-n-F

CC-n-V1

CCQG-n-F

CQCU-n-F

Dec-U-n-F

CWCU-n-F

CWCG-n-F

CCOC-n-m

CPTU-n-F

GPTU-n-F

PQU-n-F

PUQU-n-F

PGU-n-F

CGZP-n-OT

CCGU-n-F

CCQG-n-F

CUQU-n-F

CCCQU-n-F

Particular preference is given to liquid-crystalline mixtures which,besides the compounds of the formula I, comprise at least one, two,three or four compounds from Table B.

TABLE C Table C shows possible dopants which are generally added to themixtures according to the invention.

C 15

CB 15

CM 21

R/S-811

CM 44

CM 45

CM 47

R/S-1011

R/S-3011

CN

R/S-2011

R/S-4011

TABLE D Stabilisers which can be added, for example, to the mixturesaccording to the invention are mentioned below.

The entire disclosure of all applications, patents and publications,cited herein and of corresponding German patent application No. 102 23061.7, filed May 24, 2002 is incorporated by reference herein.

EXAMPLES

The following examples are intended to explain the invention withoutrestricting it. Above and below, percentages are percent by weight. Alltemperatures are given in degrees Celsius. m.p. denotes melting point,cl.p. clearing point. Furthermore, C=crystalline state, N=nematic phase,S=smectic phase and I=isotropic phase. The data between these symbolsrepresent the transition temperatures. An denotes optical anisotropy(589 nm, 20° C.). The flow viscosity ν₂₀ (mm²/sec) and the rotationalviscosity γ₁ (mPa·s) were each determined at 20° C.

Example M1 CCH—5CF₃ 8.00% Clearing point [° C.]: 83.0 CCP—1F.F.F 6.00%Δn [589 nm, 20° C.]: 0.0663 CCP—2F.F.F 8.00% Δε [1 kHz, 20° C.]: 11.7CCP—3F.F.F 8.00% γ₁ [mPa · s, 20° C.]: 157 CCOC-3-3 3.00% d · Δn [20°C.]: 0.50 CCOC-3-5 3.00% Twist [°]: 90 CCOC-4-3 4.00% CCQU-1-F 8.00%CCQU-2-F 9.00% CCQU-3-F 10.00%  CCCQU-3-F 9.00% CDU-2-F 8.00% CDU-3-F8.00% CDU-5-F 8.00% Example M2 CCP—2F.F.F 8.00% Clearing point [° C.]:83.0 CCH-34 12.00%  Δn [589 nm, 20° C.]: 0.0659 DCU-3-F 6.00% Δε [1 kHz,20° C.]: 12.3 DCU-4-F 8.00% γ₁ [mPa · s, 20° C.]: 148 DCU-5-F 8.00% d ·Δn [20° C.]: 0.50 CECU-2-F 7.00% Twist [°]: 90 CECU-3-F 7.00% V₁₀ [V]:1.16 CECU-5-F 8.00% CCQU-1-F 8.00% CCQU-2-F 8.00% CCQU-3-F 10.00% CCCQU-3-F 10.00%  Example M3 CCP—2F.F.F 11.00%  Clearing point [° C.]:85.5 CCP—3F.F.F 11.00%  Δn [589 nm, 20° C.]: 0.0754 CCP—5F.F.F 6.00% Δε[1 kHz, 20° C.]: 12.0 CCZU-2-F 4.00% γ₁ [mPa · s, 20° C.]: 155 CCZU-3-F14.00%  d · Δn [20° C.]: 0.50 CCZU-5-F 4.00% Twist [°]: 90 CGU-2-F 7.00%V₁₀ [V]: 1.11 CGU-3-F 4.00% CCH—5CF₃ 3.00% CCOC-4-3 3.00% CCQU-1-F 8.00%CCQU-2-F 10.00%  CCQU-3-F 9.00% CCCQU-3-F 6.00% Example M4 CECU-2-F8.00% Clearing point [° C.]: 82.0 CECU-3-F 8.00% Δn [589 nm, 20° C.]:0.0749 CECU-5-F 8.00% Δε [1 kHz, 20° C.]: 11.3 CCP—2F.F.F 9.00% γ₁ [mPa· s, 20° C.]: 156 CCP—3F.F.F 10.00%  CCZU-2-F 3.00% CCZU-3-F 10.00% CCZU-5-F 3.00% CCQU-1-F 9.00% CCQU-2-F 6.00% CCQU-3-F 6.00% CCCQU-3-F8.00% PCH—7F 5.00% BCH—3F.F.F 7.00% Example M5 CCP—2F.F 17.00%  Clearingpoint [° C.]: 83.0 CCP—3F.F 6.00% Δn [589 nm, 20° C.]: 0.0791 CCZU-2-F3.00% γ₁ [mPa · s, 20° C.]: 100 CCZU-3-F 9.00% d · Δn [20° C.]: 0.50CCP-31 9.00% Twist [°]: 90 CCH-34 12.00%  V₁₀ [V]: 1.65 PCH—7F.F.F 8.00%CCP—3F 5.00% PCH-302 16.00%  PUQU-3-F 7.00% CCCQU-3-F 8.00% Example M6BCH—3F.F.F 18.00%  Clearing point [° C.]: 83.0 BCH—5F.F.F 10.00%  Δn[589 nm, 20° C.]: 0.1032 BCH—2F.F 9.00% γ₁ [mPa · s, 20° C.]: 158BCH—3F.F 9.00% d · Δn [20° C.]: 0.50 BCH—4F.F 5.00% Twist [°]: 90CCP—2F.F 8.00% V₁₀ [V]: 1.27 DCU-3-F 3.00% DCU-4-F 4.00% DCU-5-F 9.00%CCP-31 7.00% CCH-34 12.00%  CCCQU-3-F 6.00% Example M7 CCH-34 5.00%Clearing point [° C.]: 79.0 PCH—5Cl 8.00% Δn [589 nm, 20° C.]: 0.0923CCP—3F.F 13.00%  γ₁ [mPa · s, 20° C.]: 160 CCP—2F.F.F 9.00% d · Δn [20°C.]: 0.50 CCP—4F.F.F 6.00% Twist [°]: 90 BCH—2F.F 7.00% V₁₀ [V]: 1.22BCH—3F.F.F 15.00%  BCH—5F.F.F 7.00% DCU-3-F 3.00% DCU-4-F 5.00% DCU-5-F8.00% CCCQU-3-F 7.00% CCP-3 1 7.00% Example M8 CCH-34 6.00% Clearingpoint [° C.]: 80.0 PCH—5Cl 8.00% Δn [589 nm, 20° C.]: 0.0932 CCP—3F.F14.00%  γ₁ [mPa · s, 20° C.]: 135 CCP—4F.F 13.00%  d · Δn [20° C.]: 0.50CCP—2F.F.F 9.00% Twist [°]: 90 CCP—4F.F.F 8.50% V₁₀ [V]: 1.25 BCH—2F.F4.00% CCCQU-3-F 8.00% CCP-31 9.00% PUQU-2-F 6.00% PUQU-3-F 8.00%PUQU-5-F 6.50% Example M9 CCP—2F.F 4.50% Clearing point [° C.]: 78.5CCP—3F.F 13.00%  Δn [589 nm, 20° C.]: 0.0797 CCP—2F.F.F 6.50% γ₁ [mPa ·s, 20° C.]: 125 PCH—7F.F.F 9.00% d · Δn [20° C.]: 0.50 CCZU-2-F 3.00%Twist [°]: 90 CCZU-3-F 10.00%  V₁₀ [V]: 1.24 CCZU-4-F 3.00% DCU-3-F2.50% DCU-4-F 5.00% PUQU-2-F 6.00% PUQU-3-F 2.50% CCP-3F 3.00% CCP-319.00% CCH-34 5.00% PCH-302 10.00%  CCCQU-3-F 8.00% Example M10 CCP—3F.F12.50%  Clearing point [° C.]: 80.0 CCP—2F.F.F 5.00% Δn [589 nm, 20°C.]: 0.080 PCH—7F.F.F 8.00% γ₁ [mPa · s, 20° C.]: 140 CCZU-2-F 3.00% d ·Δn [20° C.]: 0.50 CCZU-3-F 10.00%  Twist [°]: 90 CCZU-4-F 3.00% V₁₀ [V]:1.25 DCU-3-F 3.00% DCU-4-F 5.00% DCU-5-F 8.00% BCH—3F.F.F 9.50% CCP-319.00% CCH-34 5.00% PCH-302 11.00%  CCCQU-3-F 8.00% Example M11 BCH—3F.F10.80%  Clearing point [° C.]: 107.5 BCH—5F.F 9.00% Δn [589 nm, 20° C.]:0.0970 ECCP—30CF₃ 4.50% ν [kHz, 20° C.]: 5.9 ECCP—50CF₃ 4.50% CBC—33F1.80% CBC—53F 1.80% CBC—55F 1.80% PCH—6F 7.20% PCH—7F 5.40% CCP—20CF₃7.20% CCP—30CF₃ 10.80%  CCP—40CF₃ 6.30% CCP—50CF₃ 9.90% PCH—5F 9.00%CCCQU-3-F 10.00%  Example M12 BCH—3F.F 10.91%  Clearing point [° C.]:106.0 BCH—5F.F 9.09% ECCP—30CF₃ 4.55% ECCP—50CF₃ 4.55% CBC—33F 1.82%CBC—53F 1.82% CBC—55F 1.82% PCH—6F 7.27% PCH—7F 5.45% CCP—20CF₃ 7.27%CCP—30CF₃ 10.91%  CCP—40CF₃ 6.36% CCP—50CF₃ 10.00%  PCH—5F 9.09%CCCQU-3-F 9.10% Example M13 CCH-301 12.59%  Clearing point [° C.]: 97.5CCH—3CF₃ 7.20% Δn [589 nm, 20° C.]: 0.0639 CCH-501 9.89% ν [kHz, 20°C.]: 7.3 CCP—2F.F.F 8.99% CCP—3F.F.F 11.69%  CCP—5F.F.F 4.50% CCPC-332.70% CCZU-2-F 4.50% CCZU-3-F 15.29%  CCZU-5-F 4.50% CH-33 2.70% CH-352.70% CH-43 2.70% CCCQU-3-F 10.06% 

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. A liquid-crystalline medium comprising a mixture of polar compounds of positive dielectric anisotropy, wherein the medium comprises one or more compounds of the formula I

in which R¹ is a halogenated or unsubstituted alkyl or alkoxy radical having from 1 to 15 carbon atoms, where one or more CH₂ groups in these radicals are optionally, independently of one another, replaced by —C≡C—, —CH═CH—, —O—, —CO—O— or —O—CO— in such a way that O atoms are not linked directly to one another, X is F, Cl, CN, SF₅, a halogenated alkyl radical, a halogenated alkenyl radical, a halogenated alkoxy radical or a halogenated alkenyloxy radical having 1 to 6 carbon atoms, and L¹ and L² are each, independently of one another, H or F, and further comprises one or more compounds of the formulae O1 and O2:

in which alkyl and alkyl* are each, independently of one another, a straight-chain or branched alkyl group having 1-7 carbon atoms.
 2. A liquid-crystalline medium according to claim 1, which comprises one, two or more compounds of the formulae I-1 to I-9:

in which R¹ is as defined in claim
 1. 3. A liquid-crystalline medium according to claim 1, which further comprises one or more compounds selected from the group consisting of those of the formulae II, III, IV, V and VI:

in which the individual radicals have the following meanings: R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having 1 to 9 carbon atoms, X⁰ is F, Cl, halogenated alkyl, halogenated alkenyl, halogenated alkenyloxy or halogenated alkoxy having 1 to 6 carbon atoms, Z⁰ is —C₂F₄—, —CF═CF—, —C₂H₄—, —(CH₂)₄—, —OCH₂—, —CH₂O—, —CF₂O— or —OCF₂—, Y¹ to Y⁴ are each, independently of one another, H or F, and r is 0 or
 1. 4. A liquid medium according to claim 2 which further comprises one or more compounds selected from the group consisting of those of the formulae II, III, IV, V and VI:

in which the individual radicals have the following meanings: R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having 1 to 9 carbon atoms, X⁰ is F, Cl, halogenated alkyl, halogenated alkenyl, halogenated alkenyloxy or halogenated alkoxy having 1 to 6 carbon atoms, Z⁰ is —C₂F₄—, —CF═CF—, —C₂H₄—, —(CH₂)₄—, —OCH₂—, —CH₂O—, —CF₂O— or —OCF₂—, Y¹ to Y⁴ are each, independently of one another, H or F, and r is 0 or
 1. 5. A liquid-crystalline medium according to claim 3, wherein the proportion of compounds of the formulae I to VI together in the mixture as a whole is at least 50% by weight.
 6. A liquid-crystalline medium according to claim 1, which further comprises one or more compounds of the formulae E-a to E-d:

in which R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having 1 to 9 carbon atoms.
 7. A liquid-crystalline medium according to claim 1, which further comprises one or more compounds of the formulae IIa to IIg

in which R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having 1 to 9 carbon atoms.
 8. A liquid-crystalline medium according to claim 1, which further comprises one or more dioxane compounds of the formulae D1 and/or D2:

in which R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having 1 to 9 carbon atoms.
 9. A liquid-crystalline medium according to claim 1, wherein the proportion of compounds of the formula I in the mixture as a whole is from 0.5 to 40% by weight.
 10. An electro-optical liquid-crystal display containing a liquid-crystalline medium according to claim
 1. 11. A liquid-crystalline medium according to claim 2, wherein the medium comprises one or more compounds of the formulae I-1, I-2, I-3 or I-4.
 12. A liquid-crystalline medium according to claim 2, wherein the medium comprises one or more compounds of the formulae I-1 or I-2.
 13. A liquid-crystalline medium according to claim 6, wherein the proportion of the compounds of the formulae E-a to E-d in the medium is 10-30% by weight.
 14. A liquid-crystalline medium according to claim 1, wherein the proportion of the compounds of the formulae O1 and/or O2 in the medium is 5-10% by weight.
 15. A liquid-crystalline medium according to claim 1, wherein the medium comprises 5-35% by weight of a compound of the formula IVa:

in which the individual radicals have the following meanings: R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having 1 to 9 carbon atoms, X⁰ is F, Cl, halogenated alkyl, halogenated alkenyl, halogenated alkenyloxy or halogenated alkoxy having 1 to 6 carbon atoms.
 16. The medium of claim 15, which comprises one, two or three compounds of the formula IVa in which X⁰ is F or OCF₃. 